1. Field of the Invention
The invention relates to a liquid crystal projector apparatus and more particularly to a liquid crystal projector apparatus adapted to prevent deterioration in contrast and uniformity due to properties of dependence on viewing angles.
2. Description of the Related Art
A projector apparatus for projecting an image on a screen under magnification and thus implementing a large screen is in widespread use as an image display apparatus for indoor and outdoor use. The projector apparatus is broadly divided into a projector apparatus (a CRT projector apparatus) for projecting exiting light from a fluorescent screen of a CRT onto a screen and a projector apparatus for projecting modulated light onto a screen after modulating light emitted from a light source by means of a spatial light modulator, and, in the case of the latter, a projector apparatus (a liquid crystal projector apparatus) using a liquid crystal panel as the spatial light modulator is in the mainstream.
FIG. 1 shows an example of a configuration of an optical system of a liquid crystal projector apparatus of the related art (an optical system of a three-panel transmission type liquid crystal projector apparatus using three transmission type liquid crystal panels for RGB).
A light source 11 includes a lamp 12 (e.g., a xenon lamp), and a reflector 13 for reflecting light (white unpolarized light) emitted from the lamp 12 so as to form the light into a bundle of rays having a predetermined angle of divergence. Light emitted from the light source 11 passes through microlens arrays 14 and 15 and a polarizer 16 in sequence.
The microlens arrays 14 and 15 comprise an array of a plurality of microlenses 14a and an array of a plurality of microlenses 15a, respectively (for example, each lens has a diameter of about 1 mm to 5 mm). Each lens 14a is rectangular and similar in shape to a panel surface of each of transmission type liquid crystal modules 28, 29 and 30 in order that light exiting from the lens 14a may be focused on a panel surface of each of liquid crystal panels 42 to be described later.
The microlens array 15 is located substantially on a focal point of the lenses 14a. One each of the lenses 15a corresponds to one each of the lenses 14a, and each lens 15a has such a shape that the most possible light exiting from the corresponding lens 14a can enter into the lens 15a. 
The microlens arrays 14 and 15 allow the emitted light from the light source 11 to uniformly enter into the panel surface of the liquid crystal panel, thereby serving to improve uniformity of an image to be displayed on a screen (the uniformity refers to the uniformity of brightness or color for displaying an image of the same brightness or color over the overall screen).
The polarizer 16 is a device for converting most of incoming unpolarized light into linearly polarized light (e.g., p-polarized light) and then allowing the linearly polarized light to exit. The polarizer 16 serves to improve the efficiency of utilization of the emitted light from the light source 11 and also serves to improve contrast of an image to be displayed on a screen by increasing the quantity of exiting light from the liquid crystal panel at the time of white display.
The p-polarized light exiting from the polarizer 16 is focused and impinges on a dichroic mirror 18 by a lens 17. For example, the dichroic mirror 18 transmits red light of RGB light and reflects green light and blue light. Red p-polarized light passing through the dichroic mirror 18 is reflected by a mirror 19, and the reflected light is focused on and enters into a liquid crystal module 28 by a lens 20.
Green p-polarized light and blue p-polarized light reflected by the dichroic mirror 18 impinge on a dichroic mirror 21. For example, the dichroic mirror 21 transmits blue light and reflects green light. The green p-polarized light reflected by the dichroic mirror 21 is focused on and enters into a liquid crystal module 29 by a lens 22.
The blue p-polarized light passing through the dichroic mirror 21 is repeatedly focused and reflected by a lens 23, a mirror 24, a lens 25, a mirror 26 and a lens 27, and then the light enters into a liquid crystal module 30.
The liquid crystal modules 28, 29 and 30 have the same configuration. FIG. 2 shows an example of a configuration of an optical system of each of the liquid crystal modules 28, 29 and 30. A sheet polarizer 41 is located close to the entry side of the transmission type liquid crystal panel 42, and a polarizer 47 is located close to the exit side of the liquid crystal panel 42. The polarizer 41 has the orientation of the axis of polarization (the axis of light transmission) which is determined so as to allow the p-polarized light to pass through the polarizer 41. Therefore, the red p-polarized light, green p-polarized light and blue p-polarized light entering into the liquid crystal modules 28, 29 and 30, respectively, pass through the polarizers 41 as they are, and enter into the liquid crystal panels 42.
The liquid crystal panel 42 is a TN (twisted nematic) liquid crystal panel, and changes the locus of a resultant electric field vector of light passing through liquid crystal molecules according to the level of a voltage applied to the liquid crystal molecules. A voltage is applied to the liquid crystal molecules of pixels of the liquid crystal panels 42 of the liquid crystal modules 28, 29 and 30 in normally white mode according to the levels of video signals for red, green and blue. For example, an active matrix drive system is adopted as a system for driving the liquid crystal panel 42.
Microlenses 44, one each of which corresponds to one each of the pixels, are provided in a substrate 43 on the entry side of the liquid crystal panel 42. The microlenses 44 are lenses for focusing light incident on the corresponding pixels on effective display area portions of the pixels (i.e., portions having no electrode, switching device and so on and thus capable of allowing light to pass through the portions). The microlenses 44 serve to substantially increase an ratio aperture of the liquid crystal panel 42 and also serve to improve the contrast of an image by increasing the quantity of exiting light from the liquid crystal panel 42 at the time of white display.
Light, which passes through a liquid crystal layer 45 of the liquid crystal panel 42 and then exits through a substrate 46 on the exit side, enters into the polarizer 47. The orientation of the axis of polarization of the sheet polarizer 47 is perpendicular to that of the polarizer 41, and therefore the polarizer 47 allows s-polarized light to pass through the polarizer 47.
Red s-polarized light, green s-polarized light and blue s-polarized light passing through the polarizers 47 of the liquid crystal modules 28, 29 and 30 enter into a dichroic prism 31 from three directions, as shown in FIG. 1. The dichroic prism 31 has a filter film 31a for transmitting green light from the liquid crystal module 29 and reflecting red light from the liquid crystal module 28 in the same direction as the green light, and a filter film 31b for transmitting green light from the liquid crystal module 29 and reflecting blue light from the liquid crystal module 30 in the same direction as the green light. The red s-polarized light, green s-polarized light and blue s-polarized light are combined into one bundle of rays by the dichroic prism 31.
The s-polarized light exiting from the dichroic prism 31 is projected onto a screen (not shown) via a projection optical system 32.
As shown in FIGS. 1 and 2, the liquid crystal projector apparatus of the related art is devised to improve the uniformity by the microlens arrays 14 and 15 provided in a lighting optical system for guiding light emitted from the light source 11 to the liquid crystal modules 28, 29 and 30, to improve the contrast by the polarizer 16 provided in the lighting optical system, and to improve the contrast by the microlenses 44 provided in the substrate 43 on the entry side of each of the liquid crystal panels 42 of the liquid crystal modules 28, 29 and 30.
A TN liquid crystal panel and an STN (supertwisted nematic) liquid crystal panel have properties of being incapable of changing the locus of a resultant electric field vector of light incident obliquely on a panel surface according to initial level (according to the level of an applied voltage). This is called properties of dependence on viewing angles.
As shown in FIG. 18, the properties of dependence on viewing angles result from tilt angles (pretilt angles) p1 and p2 of liquid crystal molecules 53 to rubbing directions 51a and 52a of the respective alignment layers of substrates 51 and 52 on the entering and exit sides of a liquid crystal panel, respectively (corresponding to the substrates 43 and 46, respectively, in FIG. 7). Due to the existence of the pretilt angles, when passing through liquid crystal molecules, light incident obliquely on a panel surface produces phase shift between extraordinary light and ordinary light in a plane parallel to the panel surface and also produces phase shift between extraordinary light and ordinary light even in a plane perpendicular to the panel surface. As a result, it becomes impossible to change the locus of a resultant electric field vector of the incident light according to initial level.
In a liquid crystal display using a TN liquid crystal panel or an STN liquid crystal panel, due to the properties of dependence on viewing angles, even in normally white mode, linearly polarized light (e.g., p-polarized light) entering obliquely into the liquid crystal panel through pixels to display black (i.e., pixels to which a voltage is applied so that liquid crystal molecules may be aligned perpendicularly to a substrate) changes into elliptically polarized light, which then exits from the liquid crystal panel through the pixels. Then, s-polarized light component of the elliptically polarized light pass through an analyzer and are then projected onto a screen (that is, black stands out), and therefore the contrast may deteriorate.
FIG. 19 illustrates the correlation between an incident angle of light to a liquid crystal panel and the degree of deterioration in contrast. The greater an incident angle θ becomes, the lower contrast CR becomes, regardless of an incident direction φ in a plane parallel to a panel surface of the liquid crystal panel.
The degree of phase shifts due to the pretilt angles changes according to a cell gap length of a liquid crystal panel (i.e., a distance between two substrates which liquid crystal molecules are sandwiched between). In the case where the cell gap lengths vary according to parts of the liquid crystal panel, even when an image of the same brightness over the overall screen should be displayed (voltages on the same level are applied to all pixels), the properties of dependence on viewing angles cause variations in the transmittance of light passing through an analyzer after entering obliquely into the liquid crystal panel and then exiting from the liquid crystal panel, according to the parts of the liquid crystal panel, and therefore the brightness of the image may become nonuniform (that is, the uniformity may deteriorate).
As described above, an image display apparatus using a liquid crystal panel may deteriorate in contrast and uniformity due to the properties of dependence on viewing angles resulting from the existence of the pretilt angles. Although a direct-vision type liquid crystal display is designed in consideration of the properties of dependence on viewing angles, a liquid crystal projector apparatus that is a projection type liquid crystal display is not so devised as to improve the contrast and uniformity in consideration of the properties of dependence on viewing angles.
The reason why the direct-vision type liquid crystal display is designed in consideration of the properties of dependence on viewing angles is as follows: the quantity of light entering obliquely into the liquid crystal panel is large because an angle of divergence of light emitted from a light source is great, and moreover a screen is often viewed obliquely, so that the light entering obliquely into the liquid crystal panel and then exiting from the liquid crystal panel reaches to the eyes.
The reason why the liquid crystal projector apparatus is designed without the consideration of the properties of dependence on viewing angles is as follows: the apparatus has been heretofore designed in consideration of only linearly polarized light incident from a direction perpendicular to a panel surface, because a bundle of substantially parallel rays has been emitted from a light source and the emitted rays have entered into the liquid crystal panel via a lighting optical system while remaining substantially parallel.
However, even the liquid crystal projector apparatus recently has had a tendency to increase a range of an incident angle of light to the liquid crystal panel by increasing the angle of divergence of light emitted from the light source or by reducing the f-number of the lighting optical system, for the purpose of displaying a brighter image.
Even in the liquid crystal projector apparatus shown in FIGS. 1 and 2, the f-number of the lighting optical system is reduced so that the incident angle of light to each of the liquid crystal panels 42 of the liquid crystal modules 28, 29 and 30 lies between about plus and minus 10 to 15 degrees, for example. Therefore, p-polarized light entering obliquely into the liquid crystal panel through pixels to display black changes into elliptically polarized light, which then exits from the liquid crystal panel through the pixels, and then s-polarized light components of the elliptically polarized light pass through the polarizer 47 and are then projected onto a screen, so that the contrast deteriorates.
When the cell gap lengths vary according to parts of the liquid crystal panels 42 of the individual liquid crystal modules 28, 29 and 30, or when the cell gap lengths of the liquid crystal panels 42 differ among the liquid crystal modules 28, 29 and 30, the brightness or color of an image to be displayed on a screen becomes nonuniform (that is, the uniformity deteriorates) even when an image of the same brightness or color over the overall screen should be displayed.
Therefore, even the liquid crystal projector apparatus recently has had to prevent deterioration in contrast and uniformity due to the properties of dependence on viewing angles.
Although the liquid crystal projector apparatus is configured so that the incident angle of light to each of the liquid crystal panels lies between about plus and minus 10 to 15 degrees, the liquid crystal projector apparatus has a much narrower range of the incident angle as compared to the direct-vision type liquid crystal display. Therefore, the deterioration in contrast and uniformity cannot be prevented even if a conventional approach for designing the direct-vision type liquid crystal display in consideration of the properties of dependence on viewing angles is adopted for the liquid crystal projector apparatus as it is.